1.0 Field of the Invention
The field of the present invention relates generally to chemical baths in which metal ions build up over a period of time and must be periodically removed, and more particularly to such systems providing for coating materials, such as metals including steel, with a paint coating via a chemical reaction, in which systems an autodeposition composition bath is periodically stabilized by removing therefrom dissolved and/or dispersed multivalent metal ions accumulated over a period of operation.
2.0 Discussion of Related Art
Autophoresis and electrophoresis are two known processes for coating objects, particularly those fabricated from metallic material, with a coating composition. The electrophoresis effect provides for electrodeposition through the use of an electric field to control the movement of charged organic molecules to a workpiece serving as one electrode of a typically two-electrode system. The magnitude of electrical current and time of application is controlled for coating the workpiece to a desired thickness. The autophoresis effect permits an autodeposition coating process to be carried out via control of the de-stabilization and deposition of high-molecular-weight negative or neutrally-charged latex polymer particles, for example, onto a workpiece having a metallic surface that is chemically treated to produce positively charged ions at the surface of the workpiece which attract the oppositely or neutrally charged particles of coating composition. The parts to be coated are typically dipped into a coating bath containing the desired coating composition. Workpieces of iron, steel, galvanized metal coated with zinc, and so forth, at least about the outer surfaces of the workpiece, can typically be coated via an autodeposition coating process.
A problem in systems carrying out an autodeposition coating process is that over a period of time metal ions having a valence of two or higher (multivalent ions), dissolve and/or disperse into the bath or autodeposition composition, increasingly reducing the effectiveness of the autodeposition coating process. As the metal ions increase in concentration in the autodeposition composition, the quality of the coatings produced on the workpieces diminishes to the point where the coating composition or autodeposition bath must be replaced, or a portion of the bath must be removed and new uncontaminated coating composition added, to reduce the concentration of the metal ions, for permitting the autodeposition coating process to continue. A number of attempts have been made in the prior art to periodically remove the metal ions from the autodeposition bath or coating composition, for providing more economic use of the coating composition bath, and avoiding the necessity of disposing of contaminated bath, with all of the environmental hazards associated therewith.
Hall et al., U.S. Pat. No. 3,839,097, issued on Oct. 1, 1974, teaches the stabilization of acidic aqueous coating compositions by removing metal ions through use of an ion exchange material, such as an ion exchange resin. The ion exchange material is regenerated periodically to restore its ion exchange capacity. To accomplish such regeneration in the ion exchange material, the metal ions therein are displaced, and replaced with cations which are replaceable by metal ions to be removed from the coating composition. In one example given, an ion exchange column packed with beads of ion exchange material was first rinsed with water to reclaim residual coating composition in the column. Deionized water is thereafter run through the column to completely rinse it out. In the example given, the beads of ion exchange material are thereafter regenerated with an aqueous solution of a strong acid, in applications where the ion exchange material includes a replaceable hydrogen ion. Although the process for stabilizing a coating composition bath is taught in this reference, and also a further process for removing metal ions from beads of ion exchange material in an ion exchange column, and then regenerating the ion exchange resin, no system is shown or described for carrying out the process. For purposes of this application, the specification of Hall et al., U.S. Pat. No. 3,839,097, is incorporated herein by reference, to the extent that such teachings are not inconsistent with teachings herein.
An application for Canadian Patent Serial No. 2,017,026, published on Apr. 17, 1991, for "METHOD FOR TREATMENT OF ELECTRODEPOSITION BATH". The Canadian application teaches a method for continuously or intermittently removing a portion of an electrodeposition bath contained in a tank 10, and passing the removed portion through an ultrafilter 16. Filtered resin, pigment, and other higher molecular weight components are returned to the bath. Only the ultrafiltrate is passed through an ion exchange column 22 to remove iron and other materials from the ultrafiltrate. The filtrate from the ion exchange column 22 is returned to the electrodeposition bath, and waste products are removed from the ion exchange column 22 and disposed of. The ion exchange column 22 is regenerated by passing sulfuric acid through the column. A system for accomplishing this is taught only in a very elementary manner.
McVey, U.S. Pat. No. 3,312,189, issued on Apr. 4, 1967, shows an apparatus for forming a chromate coating on a metal surface, such as aluminum. An aqueous acidic operating solution containing hexavalent chromium ions and contaminating anion complexes is applied to the metal surface. A fluid flow control system is used for passing controlled proportions of the treating solution through cation exchange resin, and for returning the effluent therefrom back to the treatment or operating solution. Conductivity sensors are used for measuring the electrical conductivity of the effluent, which conductivity measurements are used by a controller for increasing the proportion of solution passing through the cation exchange resin in response to a decrease in the electrical conductivity of the effluent below a predetermined incremental amount higher than the solution which does not pass through the exchange resin.
In order to satisfy a recognized need in the field of the present invention, the present inventors conceived and developed a substantially automated system for periodically removing contaminants from coating composition baths used in autodeposition processing. In designing the present system, the inventors recognized the need to provide that substantially all of the autodeposition bath or coating composition be utilized in coating parts, compared to prior systems which wasted costly quantities of the autodeposition baths due to contamination thereof after a period of use forcing disposal of the same. The present inventors further recognized the requirement to provide a system which substantially minimizes the production of waste products harmful to the environment. By designing a substantially automated system for autodeposition processing, maximum economics are obtained through the use of substantially all of the costly autodeposition bath or coating composition material.
The present inventors recognized that it is contrary to prior teachings to pass a chemical containing particulates, such as latex and pigment included in autophoretic or autodeposition baths through an ion exchange (IEX) column. They conceived the present system to accomplish this operation, and overcame the problems in the prior art such as clogging of IEX columns by autophoretic baths.
3.0 Summary of the Invention
An object of the invention is to provide an improved system for autodeposition processes.
Another object of the invention is to provide an improved system for autodeposition processes that maximizes the usage of the autodeposition bath, and minimizes the production of harmful waste products.
Yet another object of the invention is to provide a substantially automated system for stabilizing a chemical bath through use of an ion exchange column to remove metal ions from the bath on a periodic basis, and further through periodic cleansing and regeneration of the ion exchange column.
With these and other objects of the invention in mind, the present invention provides for a substantially automated system programmed for periodically stabilizing a chemical bath or an autodeposition bath by passing all or a portion of the bath through a plurality of filters and an ion exchange column, for removing metal ions and other contaminants from the bath that have accumulated therein over a period of time. The system further provides for automatically pumping deionized water from a supply tank through the ion exchange column for returning treated bath from the column back to the storage tank holding the chemical or autodeposition bath. The system periodically provides for regenerating the ion exchange column by passing a regenerant acid through the ion exchange column to remove metal ions collected by the column from the autodeposition bath. Thereafter, the column is then automatically flushed out with deionized water to remove the residual acid remaining in the ion exchange column, thereby preparing the ion exchange column for another cycle of cleansing the autodeposition bath of metal ions and contaminants. Waste water and waste regenerant acid is automatically dispensed from the system to a treatment plant, in an environmentally safe manner. In another embodiment of the invention, acid passed through the ion exchange column may be collected in a reuse tank, for reuse in regenerating the ion exchange column, to the extent possible. A controller, such as a microprocessor, for example, is programmed for controlling valving means and pumping means for circulating the autodeposition or chemical bath, the deionized water, and regenerant acid, through the system in a controlled manner. An air operated diaphragm pump is used to pump the autophoretic bath to provide low shear pumping.